Ink printing apparatuses can be used for single-color or multicolor printing of a printing substrate, for example a single sheet or a belt-shaped recording medium made of the most varied materials (paper, for example). The design of such ink printing apparatuses is known, see for example EP 0 788 882 B1. Ink printing apparatuses that operate according to the drop-on-demand (DoD) principle, for example, have a print head unit with one print head or multiple print heads in a printing unit, which print head or print heads provide(s) the nozzle units comprising ink channels and activators, wherein the activators—controlled by a printer controller—can excite ink droplets in the direction towards the recording medium, which ink droplets are drawn to the recording medium in order to apply print dots there for a print image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
The design of a print head that, for example, has nozzle units with piezoelectric activators is known from U.S. Pat. No. 7,281,778. Each nozzle unit comprises an ink channel that ends in a nozzle arranged in a nozzle plate, and provides an activator that is arranged at the ink channel. The recording medium is directed past the nozzle plate. If it should be printed, the activators provided for the printing are activated by a printer controller, which activators thereupon subject the ink in the ink channels to pressure waves via which the ejection of ink droplets from the nozzles is induced in the direction towards the recording medium.
The print heads of the print head unit must be continuously supplied with ink during the printing, which ink should advantageously be degassed. For this, the printing unit can provide a supply arrangement that, for example, can be designed according to US 2012/0140003 A1. The printing unit here provides multiple print bars having print heads, which print bars are respectively connected with a distributor. The distributors are for their part connected to a back-pressure tank in which ink is arranged under pressure so that ink can be transported into the distributor. Furthermore, the back-pressure tank is connected—via a pump and a filter—with a reservoir for ink. Ink is pumped from the reservoir into the back-pressure tank when the ink level in the back-pressure tank has dropped below a set height. For this, the ink level in the back-pressure tank can be measured. The ink in the reservoir can be supplemented from an ink bottle with the aid of a pump. The ink in the back-pressure tank can be degassed. For this, a degassing unit is provided that has a known degassing means and a pump. With the aid of the pump, ink is pumped back from the back-pressure tank via the degassing means into the back-pressure tank again, and thereby is degassed. This degassing process is implemented until the ink in the back-pressure tank has been degassed to a predetermined value.
An additional supply arrangement for ink in an ink printing apparatus is known from EP 1 938 994 B1. Here a back-pressure tank for ink is provided that is connected in the printing unit with the print heads in order to supply the print heads with ink. To supplement the ink in the back-pressure tank, this is connected via a pump with a reservoir for ink. The ink in the back-pressure tank can be degassed. For this, a degassing unit made up of a degassing means, a filter and a pump is provided. Ink from the back-pressure tank is directed back via the degassing unit into the back-pressure tank again, and is thereby degassed. The ink that is not consumed in the printing unit is directed into a sub-tank, and from there is conducted back via the degassing unit into the back-pressure tank.